1. Field of the Invention
This invention relates to a moving picture camera which is capable of zooming photography, and more particularly to a moving picture camera in which the zooming speed set by the operator is maintained.
2. Description of the Prior Art
In keeping with the recent remarkable spread of video cameras in schools, factories, hospitals and homes, high zoom magnifications such as three times, six times and ten times have become required of the phototaking lenses thereof and further, power zooming by electricity has also been expedited in zooming operation.
As a result, these phototaking lenses are becoming functionally approximate to the specification of the zoom lenses mounted on handly cameras heretofore used in radio broadcasting stations for the collection of news data or for the auxiliary photography of dramas or the like while, on the other hand, there are considerably severe requirements as to the cost thereof because they are directed to the use by the general public.
Also, in the requirement for power zoom, not the ON-OFF control as in 8 mm cinecameras but the specification in which zooming speed is continuously variable and the entire zoom area is a miximum of three seconds and a minimum of 30 seconds or 60 seconds comparable to the minimum zoom area for the broadcasting station must unavoidably be realized under limited conditions.
Varying the zooming speed may simply be realized by varying the applied voltage of the motor as shown in FIG. 1A of the accompanying drawings.
However, eliminating the irregularity of zoom torque which provides the load of the motor is practically difficult because it is limited by the manufacturing accuracy of the mechanism for transferring the movable lens for zooming and the smoothness of the control cam, and the torque is also greatly affected by the attitude of the camera when levelled at an object to be photographed.
Accordingly, during slow zooming in particular, irregularity of speed is conspicuous and there occurs an inconvenience that when there is a torque fluctuation in the vicinity of a point P in FIG. 1A, the zooming stops half-way.
In contrast, in the cameras for use in broadcasting stations, the above-noted difficulties are overcome by the use of a motor provided with a tachogenerator, and the characteristic speed-feedback by the tachogenerator and servo amplifier is such that as shown in FIG. 1B of the accompanying drawings, the stable area of speed for torque variation during low speed is wide and theoretically, nearly infinitely low speed drive becomes possible, but it is set to a certain practical range under the influence of the resolution of the tachogenerator and the stability of the circuit.
FIG. 2 of the accompanying drawings shows an example of the conventional drive circuit in which speed feedback is effected by a tachogenerator 5. By a potentiometer and a variable resistor 1 connected to a seesaw type switch for zooming operation, not shown, a direction instruction in telephoto (TELE) direction or wide angle (WIDE) direction and a speed instruction voltage are applied to an amplifier 3, but the voltage is set to zero at the neutral point of the seesaw type switch. A motor 4 operatively associated with a zooming mechanism is rotated by the output of the amplifier 3, and a voltage of the opposite phase generated by the tachogenerator 5 connected to the motor 4 is applied to an adder circuit 2, whereby any irregularity of rotation resulting from torque fluctuation of the load is absorbed to achieve the stabilization of the speed. Designated by Vcc is a constant voltage source.
A problem peculiar to the drive circuit using such a tachogenerator is the expensiveness of the tachogenerator itself and in addition, during low speed, speed fluctuation may result from the resolution of the dipole of the tachogenerator.